Hydronic heating problem

I was just refilling my system last and encountered a problem. When bleeding the second floor rads the pressure won't get high enough to actually force water through. The pressure gauge above the boiler is reading 0 psi. There was enough pressure to fill the main floor rads, but that was it unfortunately.

I will likely call someone in unless there is a straightforward obvious fix. Suggestions please,

You should have a feed water regulator that will maintain about 10 psi in the system. You need to turn on the water supply to that regulator and make sure the regulator is set high enough to put water into the system.

You should also have an expansion tank that takes up the changes in volume when the water is heated.

If you follow the pipe both ways from the fill valve you should find a device that has a screw or lever on the top for adjusting it. You might also have a backflow preventer in the fill line. If the backflow preventer is installed backwards you will not get any flow through to the boiler.

I don't see any air scoop on the system. That is a device for letting the air out so it doesn't lock up the circulation system.

Thanks for the responses. I realized that the type of expansion tank I have has a pressure reducing valve built into it with a valve at the bottom (after finding the manual). The pressure setting is adjustable by adding or releasing air from this bottom valve.

I got my bicycle pump out and pumped it up resulting in more water entering the system and now I'm up to 12 psi.

I suppose either the tank is faulty or when I recently drained the system something happened. I will keep an eye on the pressure.

The expansion tank has a bladder in it sort of like a tire tube. It needs a precharge of air so there is somewhere for the water to go when it expands. It does not supply pressure to the system. When you measured the pressure where did you measure it? Are there gauges in the pipes or the boiler? You need to adjust the pressure in the expansion tank without pressure in the system. Those tanks do wear out - basically the air bladder splits or the bracket holding it rusts, keeping it from holding air. That tank should be mostly air. If it isn't, it needs to be replaced. Also, your water pressure in the system when adding water could easily be set to 12 pounds, but if the tank isn't working, it would rise significantly when the water is heated.

Hi, I measured the pressure on the boiler pressure gauge pictured in the link above. All I know is when I added air to the expansion tank more water supply came into the system and the pressure increased. I was therefore able to bleed air out of the rads on the second floor and fill with water.

As indicated I have the instructions for it which indicate that it contains a pressure reducing valve built into it, which is factory set to 12 psi.

The expansion tank is only 2 years old, but the system was drained for the past 3 months. I would continue to appreciate responses so that I can better understand the mechanism.

With a properly sized expansion tank, the pressure will not change much as the water is heated, because it has someplace to go. If the tank is not working, it will go up radically, and probably vent from the T&P valve.